1. Field of the Invention
This invention pertains generally to apparatus for regulating the flow rate from a liquid reservoir into a patient's vascular system and more particularly to such apparatus of the type including a float chamber for isolating the flow rate from the head pressure in the reservoir.
2. Statement of the Prior Art
Devices for regulating the rate of liquid flow from a reservoir into a patient's vascular system are well known. Most commonly, such devices are used to regulate liquid flowing under the influence of gravity from a reservoir through a flexible conduit terminating in a needle or cannula disposed in the patient's vein. In their simplest form, such devices comprise an adjustable clamp or the like secured on the flexible conduit. By counting the number of drops passing through the conduit over a given period of time, the infusion rate may be determined and the clamp adjusted to the desired rate. Typically, a drip chamber is interposed in the conduit to facilitate observation of the flow rate.
One of the principal drawbacks of such basic devices is that they do not isolate the flow rate from the changing head pressure in the reservoir. Thus, where accurate dosing is required, such devices must be constantly monitored and adjusted.
In an improved class of infusion devices, a float arrangement is disposed between the reservoir and the clamp. Typically, such float arrangements include a float chamber having inlet and outlet ports which communicate, respectively, with the reservoir and the flexible conduit leading to the patient. A float disposed in the chamber is connected to valves which open and close the inlet and outlet ports in response to changes in the level of liquid in the chamber. The flow rate to the patient is thus isolated from the head pressure in the reservoir. Examples of such devices may be found in U.S. Pat. Nos. 2,090,273, 3,989,043 and in commonly assigned U.S. patent application Ser. No. 596,974 filed July 18, 1975, now abandoned, and commonly assigned copending U.S. application Ser. No. 883,261, filed Mar. 3, 1978. Other infusion devices incorporating float arrangements may be found in U.S. Pat. Nos. 2,784,733, 2,538,662, 2,844,147, 3,042,038, 3,105,511, 3,311,268, 3,667,464, 3,738,361, 3,756,233, 3,931,818 and 3,963,024.
Although such devices effectively isolate the flow rate from the reservoir, it will be apparent that the flow rate will, to some extent, be dependent on the head pressure in the float chamber. In applications requiring highly accurate dosing, changes in the flow rate resulting from variations in the head pressure in the float chamber may be unacceptable. The prior art solutions to this problem are inadequate, mostly because they are unduly complex.
As noted above, it is common to employ a pinch-type clamp along the length of flexible tubing connecting the float chamber to the needle inserted in the patient's vascular system. However, because of the memory of the flexible tubing and the resulting delays in changing the flow rate when the clamp is adjusted, it has been suggested to employ a needle valve and housing therefor in place of the clamp. See, for example, British Pat. No. 817,387 issued to Fletcher et al and U.S. Pat. No. 3,605,740 issued to Price et al. One of the problems with these arrangements is that they fail to compensate for suction forces which act on the valve as a result of liquid flow through the tube leading to the patient. Even a slight change in the spacing between the needle valve and its seat will vary the flow rate and hence reduce the accuracy of the device.
It is well known that when the float arrangement is used, the chamber must be vented to the outside to permit the escape of air upon entry of liquid into the chamber and vice versa. The desirability of disposing a filter in the air vent to prevent the entry of contaminants and the escape of liquid has been suggested. See, for example, U.S. Pat. No. 3,756,233 issued to Goldowsky and U.S. Pat. No. 3,989,043 issued to Dimeff. One of the drawbacks of the prior art arrangements is that the vent communicates with the filter through a relatively small surface area of the latter thus increasing the air resistance through the vent. This, in turn, can effect both the entry and exit of liquid in the float chamber and hence the liquid flow rate to the patient.
Various valve arrangements have been suggested for closing the outlet passage from the float chamber when the chamber is completely empty. It will be apparent that this valve must provide an air tight seal to prevent the entry of air into the patient's vascular system. While arrangements employing elastomeric valves or valve seats have been suggested to effect better seating (see, for example, U.S. Pat. No. 3,963,024 issued to Goldowsky, U.S. Pat. No. 3,105,511 issued to Murphey, and commonly assigned U.S. application Ser. No. 883,261 filed Mar. 3, 1978), prior art arrangements do not provide effective sealing when the float and attached valve are tilted relative to the valve seat. Such arrangements are somewhat improved when ribbing or the like is provided on the internal walls of the chamber for maintaining proper alignment of the float and attached valve relative to the valve seat. See, for example, U.S. Pat. No. 3,931,818 issued to Goldowsky and commonly assigned co-pending application Ser. No. 883,261 filed Mar. 3, 1978. However, because of the requirement that the float be free-sliding within the chamber, the ribs do not completely avoid valve alignment problems.